This has been an on going problem since replacing my exhaust pipe and muffler. At first some family members as well as a mechanic told me it was the transmission making this noise. I took it to a transmission shop and he told me nothing was wrong with the trans and that it shifted well. He put the car on the lift and started taping around and when he hit the cat the noise was similar to the noise I was hearing. BTW he drove a 87-91 Bonneville too and he loved his car so he definitely knew about the Bonneville trans.

So fast forward, I changed my oil and after pulling the car off the ramps, the noise stopped as if the piece of *to be determine* had fallen back and this has lasted until last Friday. The noise is back and the sound has changed. It only makes the noise when I'm in Drive and when I'm holding the brake pedal. You will notice in the video when I put the car in park the noise stops. Also when I'm driving I don't here the noise. It also robbing my horsepower a little. I believe it may be the cat.

So I went out to check to see if the heat shield was the problem and while taping on the cat I heard the noise loud and clear. It is cat, sounds like something inside of it has broken off inside and I can see how air blowing out the engine can cause whatever is inside of it to rattle around. About how much do you think it will cost to replace the cat at a shop as I don't have the tools to do this myself?

The continued rattle after a single striking blow is not typical of cat-media inside the body. It definitely sounds like the heat shield. To be sure, you should be able to get a finger nail or screwdriver blade between the heat shield and the side flange (joining the top and bottom halves) of the converter. If there is a gap; that is what is causing your rattle.